1. Field of the Invention
The invention relates to a method for etching metal oxide thin films, particularly tin oxide films.
2. Discussion of the Background
Transparent Conductors are electrically conductive thin films, typically less than 1 micron thick, which transmit a substantial percentage of energy in the visible and/or solar bands of the electromagnetic spectrum. They are used in photovoltaic devices and most visual displays of both the emissive (light generating) and passive (light modifying) types. Transparent conductors consist mainly of two types:
1. Very thin metallic conductors (100-200 .ANG. thick) such as silver and gold, and PA1 2. Non stoichiometric transparent conductive metal oxides (TCOs) optionally doped for enhanced conductivity.
Examples of TCOs include: Zinc Oxide, Indium Tin Oxide (ITO) and Tin Oxide (TO) which is SnO.sub.2 optionally but preferably doped with fluorine. The current standard for many display applications is ITO, mainly because of its ease of etching at moderately elevated temperatures in strong acid or oxidizing solutions. TO is not readily chemically etched in the fine line patterns required for modem display and photovoltaic applications. In fact, for photovoltaic applications, current practice employs laser removal of the preferred TO film. This method is slow and expensive since the material is removed (vaporized) by serial progression of the laser head. Such methodology is totally inapplicable to display devices which may have thousands of lines or regions where the TCOs need to be removed to provide electrical isolation between conductive regions of the display. So, for display fabrication, the preferred process for configuring the required film patterns is to deposit the TCO on the entire surface of the substrate (usually glass), apply an etchant resist material (mask) to those portions of the TCO that it is desired to preserve, and remove or etch the unwanted material from the surface of the substrate.
TO has a number of advantages over competing TCO's for many applications:
1. Low cost. Indium is some 37 times more costly than Tin. ITO is applied in a sputtering chamber under clean room conditions. TO is applied at temperatures above 500.degree. C. where few foreign particles survive'and may be applied on the float glass line as the glass is formed.
2. TO is very durable and less subject to damage during display fabrication. It may also be directly connected to most flat cable interconnects without additional metal deposits.
3. TO is electrically stable in the 500.degree. C. range which is encountered in some important display fabrication environments.
4. TO forms cohesive bonds with the glass and/or alkali ion barrier layers, such as SiO.sub.2 and Al.sub.2 O.sub.3 deposited betwixt the substrate and the TO layers to prevent electrolytic decomposition of the TCO. This feature eliminates any concerns about TCO adhesion to the substrate.
For these and other reasons a low cost, reliable, production-prone process for etching TO is strongly desired in the art.
The etching of TO films with metallic zinc powder and hydrochloric acid (HCl) has a long history; typically the film is covered with the powdered metal, then immersed in a bath of acid. In an improvement to this procedure by Kato and Fukai, disclosed in Japanese Patent Publication 4-69234 (Nov. 5, 1992), incorporated herein by reference, ferric chloride (FeCl.sub.3) is added to the acid bath. Other innovations encountered in a review of the prior art which are unrelated to our invention include the following, each of which have one or another defects which have prevented their generalized adoption.
1. U.S. Pat. No. 4,040,892, Sargent and Ghezzo (Aug. 9, 1977). The phosphosilicate glass mask and hot concentrated HI called for is not practical in a production environment.
2. U.S. Pat. No. 3,205,155 (Sep. 7, 1965), Van Natter. Safety and disposal problems associated with alkali metal in amalgam etching as well as high cost is the problem here.
3. U.S. Pat. No. 4,009,061 (Feb. 22, 1977), Simon. Chromium (Cr.sup.++) does not reduce TO to Tin in any reasonable time frame.
4. U.S. Pat. No. 4,750,980, Hynecek et. al. (Jun. 14, 1988); U.S. Pat. No. 4,544,444, Chang (Oct. 1, 1985) U. S. Pat. No. 5,094,978, Miyagaki et. al. (May 6, 1992). These patents use plasma etching which is costly and too slow.